bc

UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

are not disturbed.
Functions may be called recursively and the automatic variables
at each level of call are protected.
The parameters named in a function definition are treated in
the same way as the automatic variables of that function
with the single exception that they are given a value
on entry to the function.
An example of a function definition is
.DS
	define a(x,y){
		auto z
		z = x*y
		return(z)
	}
.DE
The value of this function, when called, will be the
product of its
two arguments.
.PP
A function is called by the appearance of its name
followed by a string of arguments enclosed in
parentheses and separated by commas.
The result
is unpredictable if the wrong number of arguments is used.
.PP
Functions with no arguments are defined and called using
parentheses with nothing between them: b().
.PP
If the function
.ft I
a
.ft
above has been defined, then the line
.DS
a(7,3.14)
.DE
would cause the result 21.98 to be printed and the line
.DS
x = a(a(3,4),5)
.DE
would cause the value of x to become 60.
.SH
Subscripted Variables
.PP
A single lower-case letter variable name
followed by an expression in brackets is called a subscripted
variable (an array element).
The variable name is called the array name and the expression
in brackets is called the subscript.
Only one-dimensional arrays are
permitted.  The names of arrays are permitted to
collide with the names of simple variables and function names.
Any fractional
part of a subscript is discarded before use.
Subscripts must be greater than or equal to zero and 
less than or equal to 2047.
.PP
Subscripted variables may be freely used in expressions, in
function calls, and in return statements.
.PP
An array name may be used as an argument to a function,
or may be declared as automatic in
a function definition by the use of empty brackets:
.DS
f(a[\|])
define f(a[\|])
auto a[\|]
.DE
When an array name is so used, the whole contents of the array
are copied for the use of the function, and thrown away on exit
from the function.
Array names which refer to whole arrays cannot be used
in any other contexts.
.SH
Control Statements
.PP
The `if', the `while', and the `for' statements
may be used to alter the flow within programs or to cause iteration.
The range of each of them is a statement or
a compound statement consisting of a collection of
statements enclosed in braces.
They are written in the following way
.DS
if(relation) statement
while(relation) statement
for(expression1; relation; expression2) statement
.DE
or
.DS
if(relation) {statements}
while(relation) {statements}
for(expression1; relation; expression2) {statements}
.DE
.PP
A relation in one of the control statements is an expression of the form
.DS
x>y
.DE
where  two expressions are related by one of the six relational
operators <, >, <=, >=, ==, or !=.
The relation ==
stands for `equal to' and != stands for `not equal to'.
The meaning of the remaining relational operators is
clear.
.PP
BEWARE of using = instead of == in a relational.  Unfortunately,
both of them are legal, so you will not get a diagnostic
message, but = really will not do a comparison.
.PP
The `if' statement causes execution of its range
if and only if the relation is true.
Then control passes to the next statement in sequence.
.PP
The `while' statement causes execution of its range
repeatedly as long as the relation
is true.  The relation is tested before each execution
of its range and if the relation
is false, control passes to the next statement beyond the range
of the while.
.PP
The `for' statement begins
by executing `expression1'.  Then the relation is tested
and, if true, the statements in the range of the `for' are executed.
Then `expression2' is executed.  The relation is tested, and so on.
The typical use of the `for' statement is for a controlled iteration,
as in the statement
.DS
for(i=1; i<=10; i=i+1) i
.DE
which will print the integers from 1 to 10.
Here are some examples of the use of the control statements.
.DS
define f(n){
auto i, x
x=1
for(i=1; i<=n; i=i+1) x=x*i
return(x)
}
.DE
The line
.DS
	f(a)
.DE
will print
.ft I
a
.ft
factorial if
.ft I
a
.ft
is a positive integer.
Here is the definition of a function which will
compute values of the binomial coefficient
(m and n are assumed to be positive integers).
.DS
define b(n,m){
auto x, j
x=1
for(j=1; j<=m; j=j+1) x=x*(n\-j+1)/j
return(x)
}
.DE
The following function computes values of the exponential function
by summing the appropriate series
without regard for possible truncation errors:
.DS
scale = 20
define e(x){
	auto a, b, c, d, n
	a = 1
	b = 1
	c = 1
	d = 0
	n = 1
	while(1==1){
		a = a*x
		b = b*n
		c = c + a/b
		n = n + 1
		if(c==d) return(c)
		d = c
	}
}
.DE
.SH
Some Details
.PP
There are some language features that every user should know
about even if he will not use them.
.PP
Normally statements are typed one to a line.  It is also permissible
to type several statements on a line separated by semicolons.
.PP
If an assignment statement is parenthesized, it then has
a value and it can be used anywhere that an expression can.
For example, the line
.DS
(x=y+17)
.DE
not only makes the indicated assignment, but also prints the
resulting value.
.PP
Here is an example of a use of the value of an
assignment statement even when it is not parenthesized.
.DS
x = a[i=i+1]
.DE
causes a value to be assigned to x and also increments i
before it is used as a subscript.
.PP
The following constructs work in BC in exactly the same manner
as they do in the C language.  Consult the appendix or the
C manuals [2,3] for their exact workings.
.DS
.ta 2i
x=y=z  is the same as	x=(y=z)
x =+ y	x = x+y
x =\- y	x = x\-y
x =* y	x = x*y
x =/ y	x = x/y
x =% y	x = x%y
x =^ y	x = x^y
x++	(x=x+1)\-1
x\-\-	(x=x\-1)+1
++x	x = x+1
\-\-x	x = x\-1
.DE
Even if you don't intend to use the constructs,
if you type one inadvertently, something correct but unexpected
may happen.
.PP
WARNING!  In some of these constructions, spaces are
significant.
There is a real difference between
x=\-y and x= \-y.
The first replaces x by x\-y and the second by \-y.
.SH
Three Important Things
.PP
1.  To exit a BC program, type `quit'.
.PP
2. There is a comment convention identical to that of C and
of PL/I.  Comments begin with `/*' and end with `*/'.
.PP
3. There is a library of math functions which may be obtained by
typing at command level
.DS
bc \-l
.DE
This command will load a set of library functions
which, at the time of writing, consists of sine (named `s'),
cosine (`c'), arctangent (`a'), natural logarithm (`l'),
exponential (`e') and Bessel functions of integer order (`j(n,x)').  Doubtless more functions will be added
in time.
The library sets the scale to 20.  You can reset it to something
else if you like.
The design of these mathematical library routines
is discussed elsewhere [4].
.PP
If you type
.DS
bc file ...
.DE
BC will read and execute the named file or files before accepting
commands from the keyboard.  In this way, you may load your
favorite programs and function definitions.
.SH
Acknowledgement
.PP
The compiler is written in YACC [5]; its original
version  was written by S. C. Johnson.
.SH
References
.IP[1]
K. Thompson and D. M. Ritchie,
.ft I
UNIX Programmer's Manual,
.ft
Fifth Edition (1974)
.IP[2]
D. M. Ritchie,
.ft I
C Reference Manual,
.ft
.IP[3]
B. W. Kernighan,
.ft I
Programming in C: A Tutorial,
.ft
.IP[4]
Robert Morris,
.ft I
A Library of Reference Standard Mathematical Subroutines,
.ft
.IP[5]
S. C. Johnson,
.ft I
YACC, Yet Another Compiler-Compiler,
.ft
.IP[6]
R. Morris and L. L. Cherry,
.ft I
DC \- An Interactive Desk Calculator,
.ft
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